388 Scientific Intelligence. 



II. Geology axd Mineralogy. 



1. On the development of Crystallization in the Igneous rocks 

 of Washoe, Nevada, icith notes on the Geology of the District / 

 by Arnold Hague and J. P. Iddixgs. Bulletin No. IV of the 

 U. S. Geol. Survey. 44 pp. 8vo. Washington, 1885. — This 

 memoir is one of the most important and wide-bearing that has 

 come from the U. S. Geological Survey. It is " the result of an 

 investigation of the extensive and well-selected lithological mate- 

 rial collected by Mr. G. F. Becker during his examination of the 

 geological lode ;" and illustrates the fact — doubted and denied 

 by many geologists — that the rock of an igneous eruption may 

 vary in crystallization from true lavas of glass-containing igneous 

 rocks in an upper or outer region, to completely crystalline kinds, 

 granite-like in texture, in a lower or inner region. 



The grand section thi'ough igneous rocks — over four miles long 

 — afforded by the Sutro tunnel, and the vertical sections in shafts 

 2,000 to 3,U00 feet deep, make the Washoe region especially 

 favorable for such an investigation. The great differences 

 between the rocks of the territory led Mr. Becker, in his report, 

 to recognize, in accordance with German teaching, two groups 

 among them, a Tertiary and a pre-Tertiary ; and the same dis- 

 tinction was made out in his study of the rocks met with on 

 descending in the mine, the lower being of the older division. 

 The specimens collected in his careful investigation of the region 

 number more than 2,000, 600 of them from the surface rocks, and 

 over 1,400 from underground. The rocks made out by him were 

 granular diorite, porphyritic diorite, micaceous diorite-porphyry, 

 quartz-porphyry, earlier-diabase, later-diabase, earlier-hornblende- 

 andesite, augite-andesite, later-hornblende-andesite, basalt. 



Messrs. Hague and Iddings, after a microscopic study of thin 

 sections of the various rocks (over 500 in number), have obtained 

 the following results. 



The study of the diabase and augite-andesite (of which there 

 were 140 thin sections) shows, first, "the absolute identity," in 

 nature and occurrence, of the mineral constituents of the two 

 rocks, so that many kinds under each are not distinguishable; 

 secondly, a gradual transition from a microlitic glassy ground- 

 mass to a holocrystalline ; and from microscopically fine-grained 

 to coarse-grained and porphyritic varieties. These two points 

 are established by finding imperceptible gradations, in all respects, 

 between the diabase and augite-andesite. A study of the succes- 

 sive rocks along the Sutro tunnel sustains the conclusions that the 

 glass-bearing rocks are the exterior; that the degree of crystal- 

 lization increases inward; that the two kinds of rocks here con- 

 sidered are for the most part indistinguishable. 



The granular-diorite, at the head of the Sutro tunnel, was 

 proved to belong with the pyroxene rocks. The same rock from 

 Mt. Davidson and Ophir Ravine was found to be identical with 

 the coarse-grained diabase. 



